In a duplex processor system, including first and second processor copies means, each processor copy means being connected to corresponding receiver/transmitter means, a circuit for controlling simplex I/O terminals by said duplex processors, said circuit comprising:
- first terminal control means connected to said first processor copy means and to said corresponding receiver/transmitter means, said first terminal control means being operated in response to a first processor signal to transmit data from said first processor copy means and to transmit data to said first processor copy means;
- second terminal control means connected to said second processor copy means and to said corresponding receiver/transmitter means, said second terminal control means being operated in response to a second processor signal to transmit data from said second processor copy means and to transmit data to said second processor copy means;
- first interface means connected to said first terminal control means and to a first simplex I/O terminal, said first interface means being operated in response to said first terminal control means to transfer said data from said first processor copy means to said first simplex I/O terminal and said first interface means being further operated to transfer said data from said first simplex I/O terminal to said first processor copy means;
- second interface means connected to said second terminal control means and to a second simplex I/O terminal, said second interface means being operated in response to said second terminal control means to transfer said data from said second processor copy means to said second simplex I/O terminal and said second interface means being further operated to transfer said data from said second I/O terminal to said second processor copy means;
- said connection of said first processor copy means to said first terminal control means including first indicating means for transmitting said first processor signal having a first value for said first processor copy means being in service and said first processor signal having a second value for said first processor copy means being out of service;
- said connection of said second processor copy means to said second terminal control means including second indicating means for transmitting said second processor signal having a first value for said second processor copy means being in service and said second processor signal having a second value for said second processor copy means being out of service;
- said first terminal control means connected to said second interface means, said first terminal control means being operated in response to said second value of said second processor signal to transmit said data of said first processor copy means to said second simplex I/O terminal and said first terminal control means being further operated to receive said data from said first simplex I/O terminal;
- said connection of said first interface means to said first terminal control means including:
- first transmitting means;
- first receiving means;
- first data terminal ready means connected to said first terminal control means and being operated to transmit to said first terminal control means a first data terminal ready signal of the first value for said first simplex I/O terminal being out of service and to transmit said first ready signal of a second value for said simplex I/O terminal being in service;
- said second terminal control means connected to said first interface means, said second terminal control means being operated in response to said second value of said first processor signal to transmit said data of said second processor copy means to said first simplex I/O terminal and said second terminal control means being further operated to receive said data from said second I/O terminal;
- said connection of said second terminal control means to said second interface means including:
- second receiving means;
- second transmitting means;
- second data terminal ready means connected to said second terminal control means and being operated to transmit to said second terminal control means a second data terminal ready signal of a first value for said simplex I/O terminal being out of service and to transmit said second ready signal of a second value for said simplex I/O terminal being in service.